Actuating and confirming device for printing electromagnets

ABSTRACT

A device is provided for commanding and confirming the operation of an electromagnet used for actuating print hammers in high speed printers. The electromagnet windings are fed through a constant current source and a threshold device detects the voltage pulse caused by armature movement, this pulse being discriminated from other pulses by suitable timing devices. The detection of the pulse causes interruption of current to the electromagnet and its absence triggers an alarm.

United States Patent 1 Vigini June 19, 1973 ACTUATING AND CONFIRMING DEVICE 3,293,505 12/1966 Miller 317/010. 4 FOR PRINTING ELECTROMAGNETS 3,295,421 1 1967 McCormick 3l7/DlG. 4

[75] Inventor: Giorgio Vigini, Milan, ltaly [73] Assignee: Honeywell Information Systems Italia, Caluso, ltaly [22] Filed: Mar. 10, 1972 [2]] Appl. No.: 233,660

[30] Foreign Application Priority Data Mar. 20, 1971 Italy 22041 A/7l [52] US. Cl.3l7/l48 .5 R, 3l7/DlG. 4, 317/DlG. 6,

[51] Int. Cl. 01h 47/32 [58] ,Field of Search 3l7/DIG. 4, DlG. 6,

[56] References Cited UNITED STATES PATENTS 3/1964 Marshall 3l7/DIG. 6

Primary Examiner--J. D. Miller Assistant Examiner-Harry E. Moose, Jr.

Attorney- Aubrej C. Brine, Fred Jacob and Ronald T.

Reiling et al.

7 Claims, 4 Drawing Figures Patented June 19, 1973 3,740,615

2 Shuts-Sheet 1 FIG. 2

Patented June 19, 1973 2 Shuts-Sheet 2 BACKGROUND OF THE INVENTION The present invention relates to a device for commanding the operation of an electromagnet, and for confirming its achievement; more particularly, it is related to the electromagnets used for actuating the print hammers employed in high speed printers for data processing.

In those devices where a mechanical member is actuated by an electromagnet, it may be of paramount importance to provide means for checking that the mechanical member has accomplished the required operation. In the particular case of impact printers, wherein the print operation is carried out by the impact of a hammer member actuated by an electromagnet, it is necessary to make sure that the print control signal, that is, the current pulse energizing the electromagnet, has caused the operation of the mechanical member which in turn actuates the hammer member. In most cases this mechanical member is the armature of the electromagnet.

On the other hand it is of practical interest to interrupt the current energizing the electromagnet as soon as the armature has completed its motion, to avoid excessive heating of the electromagnet windings and a useless energy output.

Arrangements are known, whereby the accomplishment of the printing operation is checked by independent detecting members sensitive to the hammer motion, such as electromagnetic, optical, or piezoelectric pick-up devices or the like.

This type of arrangement is a complex and expensive one, especially in the case of high speed parallel printers where the number of hammers is very high. In case of serial printers, wherein a single hammer is moved along the print line, the independent pick-up device, and its connection wires, increase weight, space requirement and failure probability of the hammer carrying member.

The object of the invention is to obviate such problems by providing a device for detecting the operation of the electromagnet armature, for interrupting the feeding of the same when the operation has been'carried out, and for providing an alarm signal in case of non-operation of the armature.

SUMMARY OF THE INVENTION The above object is attained, according to the inven- BRIEF DESCRIPTION OF THE DRAWINGS The following detailed description of a preferred embodiment will be better understood by referring to the attached drawings, in which:

FIG. 1 is a simplified wiring diagram of the firing circuit of the electromagnet and of the detecting circuit according to the invention;

FIG. 2 shows the time diagrams of different electrical values and of binary levels at different points of the circuit;

FIG. 3 is a simplified logical block diagram of the device according to the invention; and

FIG. 4 is a simplified logical block diagram of the device according to the invention, when applied to a parallel high speed printer.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the annexed drawings, FIG. 1 shows the simplified wiring diagram of the firing circuit (indicated as a whole by the reference numeral 1), for example, of the hammer of a serial printer; of a circuit comprising a differentiator and a threshold device, indicated as a whole by reference numeral 2, for detecting the signal indicating the accomplishment of the movement of the armature, and of a circuit 3 for regenerating and forming the output signal.

The winding EM of the electromagnet is fed by a positive source voltage +V,,, for instance +48V. The energizing current is controlled by a transistor T1 whose emitter is connected to ground through a resistor RI. The base of transistor T1 is connected to the emitter of a transistor T2, whose collector is connected to the collector of transistor T1. Resistors R2 and R3 respectively connect the emitter of T1 to its base, and equally the emitter of T2 to its base, thus forward biasing the said bases. The base of T2 is connected to the collector of a transistor T3 through a diode DI; the collector of T3 is fed by a voltage source+V through resistor R6.

By means of a diode D3 and a manual switch CM, the base of T2 inay be connected to three different voltage sources V,,, V,,, V,,, wherein V,, V,, V"-,,. For instance, V,, 3V.; V,, 4 V; V, 5 V.-

The base of T3 is connected to the collector of an input transistor T4, with grounded emitter, and with the collector fed by a fixed voltage V, for instance 12 V., through resistor R7: its base is connected to the input terminal I.

All transistors in the described embodiment are of the NPN type. The binary level ZERO is made to correspond to zero voltage, and the binary level ONE corresponds to a positive voltage suitable for controlling the operation of the transistors involved.

The logical part of the device, provided for controlling the operation of the feeding device and for handling the related signals, as described hereafter, is suitably implemented by integrated circuit units. Therefore, the positive voltage corresponding to the binary ONE has a value compatible with the standards of said integrated circuit units and in the described embodiment said positive voltage is assumed to be 2 V.

When the input terminal I is as ZERO level, that is, practically at O V., transistor T4 is Off; its collector is positive, thus maintaining transistor T3 On. Consequently, its collector is practically at 0 V., transistor T2 is Off, and as the base of T1 is also at alow voltage level, T1 is also Off. The electromagnet EM is deenergized.

If a ONE level, that is, for instance, a 2 V signal is applied to input terminal I, T4 goes On, T3 goes Off, its collector acquires a voltage near +V. The diode D3 limits the voltage of the base of T2 to the value selected by the manual switch CM, for instance V,, 4V. This voltage, applied through diode D1 to the base of T2, drives it On. Due to the voltage drop across resistor R2 the base of T1 is also positively biased with respect to the emitter, and therefore T1 is conductive. A current flows through the winding EM, the transistor T1 and the resistor R1.

The diagram of the current flowing through EM is shown by diagram i) of FIG. 2. At rest, the current is null. As soon as T1 and T2 conduct, the current starts to increase.

At the start, T1 is saturated and therefore of negligible resistance; as resistor R3 has a relatively small value, the inductance of EM is prevailing, and the current increases approximately according to the line between and II: this line is the initial part of an exponential curve having a relatively high time constant, and can be represented by a straight line.

Diagram v) indicates, by a solid thick line, the variation of the voltage of the point P1 of F IG. 1, connected to the collector of T1. As the current starts flowing, this voltage, which was, at rest, equal to V0, (for instance +48 V), goes substantially to 0V. Then, as the current flowing through R1 increases, this voltage also increases. The potential of the emitter of T1 increases with respect to the base potential, which depends on the value of Vn, thus diminishing the forward bias of the emitter-base junction, and bringing the transistor into the active region of operation. The current ceases to increase, and becomes stabilized to a constant value by the feed-back effect due to resistor R1. The circuit operates as a constant current generator.

As the current value becomes constant, the inductance of EM has no more effect, and the voltage rapidly increases up to a value V1 which differs from Vu only by the drop across the very small resistance of EM.

Therefore, at instant t1, a steep-rising front for the voltage of point P1 takes place.

As the armature'starts to move reducing the air gap, the flux increases and an increasing counterelectromotive force is generated, which is approximately proportional to the speed of the armature. This c.e.m.f. is subtracted from the voltage V1, as shown by the thick solid line of diagram v) OF FlG. 2.

At the end of its travel, that is, at instant t2, the armature is abruptly stopped, the c.e.m.f. disappears, and the voltage reverts rapidly to the V1 level. Therefore, at time instant 12, a second steep voltage front takes place.

If the manual switch CM is set on any one of the other two positions, the base voltage of transistor T1 is different: if the switch is set to position V",,, the base voltage is higher (appr. V); if it is set to V,,, it is lower (appr. 3 V). The constant 'current flowing through the electromagnet windings is, in the first case, higher, because the voltage drop through R3, needed to properly reduce the junction bias, is larger: in the second case, the current will be lower. The current value is represented, in the two-instances, by the thin solid lines i" and i' of diagram 1'); the voltage values are respectively shown by the dashed lines v" and v'. If the control current is higher, the travel time of the armature is lower, and it reaches the end position at the time t"2, preceding I2; in the opposite instance it will be the reverse, and t2 will follow r2.

This adjustment of the intensity of the energizing current of the printing electromagnet is employed in printers to adjust the energy of their print impact, according, for example to the number of copies to be printed at the same time. The variation of the energization also causes a variation in the printing time, which must be compensated, by proper artifices, only in the case of on the flight" printers, where the printing is made without stopping the type-carrying member and it has no relevance if the printing member is stopped at the moment of printing.

Reference numeral 2 in FIG. 1 indicates the differentiating and-threshold circuit. It comprises substantially a capacitor C1, two resistors R4 and R4, a diode D2 and a Zener diode Z, connected to point P, and an output resistor R5.

The diode D2 holds the point P2 to a potential of substantially 0 V when the potential of point P1 decreases, for instance as the transistor T1 starts to conduct. On the other hand, when this potential is abruptly increasing, at instant t1 and t2, the diode D2 is reverse biased and the potential of P2 increases rapidly supplying a positive pulse, as indicated by diagram p) of FIG. 2. Such pulses are transferred, through the Zener diode Z, to the terminals of resistor R5. The Zener diode Z provides a voltage threshold, whose value is represented by the dashed line Z in diagram p), so that only the pulses having an amplitude greater than the said threshold value, such as the pulses which take place at times I! and t2, are transmitted to the terminal P2 and thereby to the following circuit. Noise pulses, of lower value are not transmitted to the output P2.

Point P3 is the input terminal of a pulse forming and amplifying circuit, indicated by reference numeral 3 and comprising the transistors T5 and T6, which have grounded emitters and the collectors fed by voltage source +V through resistors R8 and R9 respectively.

The base of the transistor T5 is connected to the central point of a voltage divider comprising the resistors R10 and R11 and its collector is connected to the base of transistor T6 whose collector is connected directly to output terminal U, and to the ground through resistor R12. The value of voltage V, for instance 5 V, and the characteristics of the output voltage divider comprising the resistors R9 and R12 are so chosen, that the amplitude of the voltage swing at the output U between the values corresponding to binary levels 0 and l respectively, iscompatible with the standards of the adopted integrated circuits; for instance, these values, may be respectively 0V. and 2V.

A resistance-capacity network, comprising the resistors R13 and R14, series connected between the base of T5 and the output terminal, and the capacitor C2, parallel connected to R4, contributes to suitably shape the output pulses, which are shown in diagram u)of FIG. 2.

FIG. 3 shows the block diagramof the logical device according to the invention. it is assumed to be formed by integrated circuit units according to the standard system called TTL (Transistor Transistor Logic), and comprises the one-shots ll, 13, 16, provided with a set input terminal S, a reset" input terminal R, a dir ect output terminal Q and an inverted output terminal verted. It returns to rest after a predetermined reset delay. If input R goes to a 0" level, the one-shot is reset to the rest state and remains so even if a negative-going front is applied to the S terminal.

Other logical units used in this circuit are two-input NAND gates 15 and 17. The output of such a gate is 0 if, and only if, both the inputs are at binary level l." Further, there is a bistable multivibrator (flipflop) 18 having an inverted .T for setting and a direct input K for resetting. It is set to work state when a 0 level is applied to input I whatever is the level applied to K.

In the block diagram of FIG. 3 the three blocks enclosed by the dashed line, and indicated by 1, 2 and 3 are intended to enclose the circuits indicated by the same reference numerals in FIG. 1, including the feeding terminals, the manual switch CM, the electromagnet EM and the ground leads.

The command for firing the printing hammer is applied to input as a short pulse of binary level I.

Input 10 is connected directly to the inputs S of the three one-shots ll, 13, 16.

The one-shot 13 is adjusted to a reset delay comprised between the time intervals t0 t1 and t0 t2. Its inverted output is connected to one of the inputs of the NAND gate 15, whose second inputs are connected to the output U OF the pulse-shaping circuit 3. If, for example, the interval t0 I1 is equal to 900 us, and the interval :0 22 is equal to 1,400 MS, the reset delay of output 11 may be of 1,200 us. The one-shot 11 is adjusted to a relatively long reset delay, for example 5 ms. Its direct output is connected to input I of the hammer firing circuit, and also to one of the inputs of NAND gate 17, whose output is connected to the input J of the flip-flop 18.

The one-shot 16 is adjusted to a reset delay which also is relatively long, but not so long as that of oneshot 16, for instance 3.5 ms. Its inverted output is connected to the second input of NAND gate 17.

The hammer firing command pulse, applied to input 10, controls, by its falling front, the setting of the three one-shots ll, 13 and 16.

As the one-shot 11 is set to work state, its direct output supplies a ONE level to input I of the firing circuit 1. As a consequence, two pulses u of binary level ONE will appear to the output U of the pulse forming circuit 3, at times 11 and r2, as represented in diagram 11) of FIG. 2.

These pulses are supplied, in succession, at said times, to an input of the NAND gate 15. The setting of one-shot 13 causes a ZERO level to appear at the inverted output of the same. As this inverted output is connected to the second input of NAND gate 15, the output of the gate is 1" as long as the one-shot is not reset, that is, at least until time 14 (diagram ql of FIG. 2)

The first pulse supplied by circuit 3 cannot therefore be transferred to the output of this NAND gate. At time t4 the one-shot 13 returns to rest, and the second input of NAND gate 15 becomes ONE." Then the second pulse supplied by the circuit 3 causes the NAND gate 15 to supply, at its output, a short 0" level pulse, which applied to the reset input of the one-shot 11, causes it to return to rest. The direct output O of this one-shot goes to O, and this level, applied to the input I of the firing circuit, causes, as stated, the transistor T1 to go Off and therefore the electromagnet EM to be deenergized. Thus, the signal informing that the armature has properly operated causes the electromagnet to be de-energized, preventing any further flowing of current and therefore, any unnecessary heating of the electromagnet winding, and of the control circuit. At the same time t2 the voltage of point P1 returns to V0 (FIG. 2).

The setting in the the work state of the one-shot 16 at time t0 supplies a binary 0" to its inverted output 6, and therefore, to a first input of NAND gate 17. As long, at least as the one-shot 16 remains in the work state, that is, at least, until time t5, the output of the NAND gate 17 is l (diagram :13 of FIG. 2).

However, if, for any reason, the armature of the electromagnet has not been attracted, the output signal of circuit 3 is lacking, the reset input of the one shot 11 does not receive the reset signal and its output remains at level 1 for the whole time interval until the oneshot 11 is reset, that is, until time t6 (dashed line in diagram Q2 of FIG. 2). Therefore, at time :5, when the one-shot 16 resets, and supplies a level l also to the second input of NAND gate 17, this output goes to 0" and causes setting in the work state of the flip-flop 18, whose output triggers an alarm signal. Subsequently, at time t6, that is after 5 ms. the one-shot 11 is reset and removes the feeding source from the electromagnet.

It may be remarked that, if the armature is attracted, but for any accidental cause its motion happens to be relatively slow, thus actuating the hammer with insuffi cient energy, the rising front of voltage V will not be steep enough to transfer a pulse across capacitor C of sufficient amplitude to overcome the threshold device formed by Zener diode Z: therefore there will be no pulse at the output U, and a signal of defective'operation will be originated.

Moreover, as it appears from diagram v) of FIG. 2, energizing of the electromagnet is always interrupted as soon as the printing has taken place. This is useful in the case in which the printing instant changes according to the energy of the electromagnet energizing pulse,

as may happen in case of printing by means of a standing typecarrier member.

If the device according to the invention is lacking, the energizing of the electromagnet must be maintained for a time sufficient to allow the printing to take place even in the case of minimum energy of the pulse, and of maximum delay or operation of the hammer, that is, until after r2. This fact, in case of maximum energy, and minimum delay, means useless current output during the time interval t2 t'2, and a higher heating of the components.

FIG. 4 shows schematically how the device may be applied to a parallel printer. In this instance, it is known that the printer is provided with a plurality of electromagnets EM 1, EM2 EMn, each one of them operating a hammer, for each print position. A typecarrying member, in continuous motion, keeps all characters passing in front of all print positions. A control device applies a print command signal, which is for example a binary 1 pulse, to the control terminals TCl, TC2 TCn of the electromagnets, at the proper instants, so selected, that at the moment of printing, the character to be printed is in front of the operated hammer.

As known, the typecarrying member maybe a drum, carrying all the characters arranged in parallel columns and kept in continuous rotation, or a chain, or a flexible ribbon moving steadily in a direction parallel to the print line, or also a rigid bar in alternating motion along the same direction. In any case, when the typecarrying member has moved the whole set of characters in front of all the printing hammers, the printing of a' line is completed: then the printing operation is stopped for the time needed for operating the vertical feeding of the paper at least for the space of a line interval (line feed). As the current needed by a parallel printer is considerable, it is useful to interrupt the energization of each electromagnet as soon as the printing has been effected by the same.

According to the invention, the electromagnets are fed in parallel by an energy source +VC and the operation of each one of them is controlled by an individual control device DCI, DC2, DCn.

These control devices are the same as those shown by FIG. 1.

For each one of these control devices, only a part of the feeding circuit 1, of FIG. 1, already described, is indicated in FIG. 4.

The threshold circuit 2 and the pulse shaper 3 of FIG. 1 are assumed to be comprised in the dashed line box 4 of FIG. 4.

Each one of these control circuits is provided with an input terminal ICl, IC2, ICn and an output terminal UCl, UC2, .UCn. Each circuit causes the energizing of the associated electromagnet when a binary ONE pulse is applied to the one correspondent input IC.

When the associated armature has completed its travel, each output UC supplies a binary ONE pulse.

A flip-flop FCl, FC2, FCn, having a set input terminal S and a reset input terminal R, is associated with each one of these devices, these inputs being activated by a binary ONE level. Such flip-flops have a direct output Q which is at level at rest and at level 1" at work, and an inverted output 1 assuming the opposite levels. The input S of each flip-flop is connected to the input terminal 1c of the associated control circuit DC.

The output UC of each control device is connected to the reset input R of the same flip-flop.

When the control device supplies a print command pulse to a terminal TC, this pulse sets the flip-flop in the work state, and the output Q applies a level ONE to the input IC of the control device, causing the energization of the associated electromagnet. In the normal operating condition, the completion of the armature travel causes a pulse of level ONE to appear at output UC: this pulse, applied to the reset input of the associated flip-flop resets the same in rest condition, and interrupts the energizing of the electromagnet.

If all the characters of a line are correctly printed, at the end of the line print all the direct outputs of the flipflop FC have returned to 0" level. In the contrary case, at least one of them is at l level and this peculiarity will be signalled in a suitable manner. For example, the flip-flops FC may be subdivided into in groups, for example comprising eight flip-flops each, and the inverted outputs of the flip-flops of each group may be connected to an eight-input NAND gate, as those indicated by NCl NCZ, NCm in FIG. 4. The output of each NAND gate is connected to the set input of flipflops FAl, FA2, FAm. If an input of these NAND gates is at 0 level, which means that the armature of the related electromagnet did not correctly complete the travel, the output of the related NAND gate is l This value sets the related flip-flop FA to the work state, which output triggers an alarm signal and at the same time identifies the group of eight electromagnets which comprises the defective one. Other means, known in the art, may be used to detect the failure and identify the defective electromagnet, such as that of scanning the outputs of the flip-flops FC, during the line feed interval, to detect which one is in the work condition. In this case the defective electromagnet may be easily identified. Another method is to connect all the direct outputs, by a diode OR circuit, to the input of a single supervising flip-flop in which case the failure is detected, but the defective electromagnet is not identified.

What is claimed is 1. A device for actuating an electromagnet and con firming the operation thereof, comprising:

a constant current circuit for feeding said electromagnet,

control means for energizing said electromagnet by said feeding circuit in response to an actuating signal,

circuital means for detecting a voltage swing at a terminal of said electromagnet due to the abrupt stopping of the motion of the armature thereof, and for delivering a confirming signal in response to said voltage swing.

2. The device of claim 1, wherein said detecting means comprise amplitude discriminating means having a definite threshold level, through time discriminating means having a definite threshold and time discriminating means for accepting signals comprised in predetermined time intervals.

3. The device of claim 1, wherein said confirming signal causes the feeding of the electromagnet to be interrupted.

4. In an impact printer, a device for actuating a printing electromagnet in response to an actuating signal and for confirming the operation thereof, comprising:

a direct current voltage source, having one pole connected to a first terminal of the winding of said electromagnet, and the other pole connected to a common reference lead,

current control means, comprising a transistor having the collector connected to a second terminal of said winding, the base connected to circuital means for driving said transistor into conduction in response to a switching signal depending on said actuating signal, the emitter connected to said common reference lead through a first resistor for generating a negative feedback to the effect of maintaining substantially constant the current energizing said electromagnet, whenever said transistor is in conductive condition,

a differentiating circuit comprising a capacitor connected to said second terminal of said winding, a second resistor and a diode serially connected between said capacitor and said common reference lead,

threshold means comprising a Zener diode and a third resistor serially connected between the anode of said diode and said common reference lead,

for generating a confirming pulse in response to a voltage swing of predetermined characteristics of said second terminal of said winding, due to the stopping of the motion of the electromagnet armature.

5. The device of claim 4, comprising, in'addition, a

first one-shot circuit being set in a first unstable state by said actuating signal applied to a set terminal thereby delivering said switching signal at an output terminal, and being adapted to be reset in a second stable state by said confirming pulse applied to a reset terminal.

6. In an impact printer, a device for actuating a printing electromagnet in response to an actuating signal and for confirming the operation thereof, comprising:

a direct current voltage source, having one pole connected to a first terminal of the winding of said electromagnet, and the other pole connected to a common reference lead,

current control means, comprising a transistor having the collector connected to a second terminal of said winding, the base connected to circuital means for driving said transistor into conduction in response to a switching signal depending on said actuating signal, the emitter connected to said common reference lead through a first resistor for generating a negative feedback to the effect of maintaining substantially constant the current energizing said electromagnet, whenever said transistor is in conductive condition,

a differentiating circuit comprising a capacitor connected to said second terminal of said winding, a second resistor and a diode serially connected between said capacitor and said common reference lead,

threshold means comprising a Zener diode and a third resistor serially connected between the anode of said diode and said common reference lead for generating a confirming pulse in response to a voltage swing of predetermined characteristics of said second terminal of said winding, due to the stopping of the motion of the electromagnet armature, a first one-shot circuit being set in a first unstable state by said actuating signal applied to a set terminal thereby delivering said switching signal at an output terminal, and being adapted to be reset in a second stable state by said confirming pulse applied to a reset terminal,

a second one-shot circuit being set in a first unstable state by said actuating signal, and reverting a second stable state after a delay suitably shorter than the minimum operation time of the electromagnet, and gating means preventing said confirming pulse from reaching the reset terminal of said first oneshot circuit as long as said second one-shot circuit is in said first unstable state.

7. The device of claim 6, comprising, in addition, a third one-shot circuit being set in a first unstable state by said actuating signal, and reverting to a second stable state after a delay suitably shorter than the delay after which said first one-shot circuit reverts in the second stable state in absence of said confirming pulse, and gating and switching means for generating an alarm signal in case there is a coincidence of the first unstable state of the first one-shot circuit and of the second stable state of the third one-shot circuit. 

1. A device for actuating an electromagnet and confirming the operation thereof, comprising: a constant current circuit for feeding said electromagnet, control means for energizing said electromagnet by said feeding circuit in response to an actuating signal, circuital means for detecting a voltage swing at a terminal of said electromagnet due to the abrupt stopping of the motion of the armature thereof, and for delivering a confirming signal in response to said voltage swing.
 2. The device of claim 1, wherein said detecting means comprise amplitude discriminating means having a definite threshold level, through time discriminating means having a definite threshold and time discriminating means for accepting signals comprised in predetermined time intervals.
 3. The device of claim 1, wherein said confirming signal causes the feeding of the electromagnet to be interrupted.
 4. In an impact printer, a device for actuating a printing electromagnet in response to an actuating signal and for confirming the operation thereof, comprising: a direct current voltage source, having one pole connected to a first terminal of the winding of saId electromagnet, and the other pole connected to a common reference lead, current control means, comprising a transistor having the collector connected to a second terminal of said winding, the base connected to circuital means for driving said transistor into conduction in response to a switching signal depending on said actuating signal, the emitter connected to said common reference lead through a first resistor for generating a negative feedback to the effect of maintaining substantially constant the current energizing said electromagnet, whenever said transistor is in conductive condition, a differentiating circuit comprising a capacitor connected to said second terminal of said winding, a second resistor and a diode serially connected between said capacitor and said common reference lead, threshold means comprising a Zener diode and a third resistor serially connected between the anode of said diode and said common reference lead, for generating a confirming pulse in response to a voltage swing of predetermined characteristics of said second terminal of said winding, due to the stopping of the motion of the electromagnet armature.
 5. The device of claim 4, comprising, in addition, a first one-shot circuit being set in a first unstable state by said actuating signal applied to a set terminal thereby delivering said switching signal at an output terminal, and being adapted to be reset in a second stable state by said confirming pulse applied to a reset terminal.
 6. In an impact printer, a device for actuating a printing electromagnet in response to an actuating signal and for confirming the operation thereof, comprising: a direct current voltage source, having one pole connected to a first terminal of the winding of said electromagnet, and the other pole connected to a common reference lead, current control means, comprising a transistor having the collector connected to a second terminal of said winding, the base connected to circuital means for driving said transistor into conduction in response to a switching signal depending on said actuating signal, the emitter connected to said common reference lead through a first resistor for generating a negative feedback to the effect of maintaining substantially constant the current energizing said electromagnet, whenever said transistor is in conductive condition, a differentiating circuit comprising a capacitor connected to said second terminal of said winding, a second resistor and a diode serially connected between said capacitor and said common reference lead, threshold means comprising a Zener diode and a third resistor serially connected between the anode of said diode and said common reference lead for generating a confirming pulse in response to a voltage swing of predetermined characteristics of said second terminal of said winding, due to the stopping of the motion of the electromagnet armature, a first one-shot circuit being set in a first unstable state by said actuating signal applied to a set terminal thereby delivering said switching signal at an output terminal, and being adapted to be reset in a second stable state by said confirming pulse applied to a reset terminal, a second one-shot circuit being set in a first unstable state by said actuating signal, and reverting a second stable state after a delay suitably shorter than the minimum operation time of the electromagnet, and gating means preventing said confirming pulse from reaching the reset terminal of said first one-shot circuit as long as said second one-shot circuit is in said first unstable state.
 7. The device of claim 6, comprising, in addition, a third one-shot circuit being set in a first unstable state by said actuating signal, and reverting to a second stable state after a delay suitably shorter than the delay after which said first one-shot circuit reverts in the second stable state in absence of said confirming pulse, and gating and switching means for generating an alarm signal in case there is a Coincidence of the first unstable state of the first one-shot circuit and of the second stable state of the third one-shot circuit. 